You know, Undertale and In Stars and Time are very interesting games to compare, even outside of being Eartbound-Inspired-RPGS-TM that rewrote my entire personality, mostly because of their contrasting approaches to morality.
PS - SPOILERS FOR BOTH! IF YOU HAVEN'T FINISHED EITHER GAME, DROP THIS LIKE THE PLAGUE AND KEEP SCROLLING!!
Undertale as a game is very interested in the ethical implications of player choices: all the monsters are functionally people, and killing them is treated like murder in a direct subversion of RPG tropes. Each death holds narrative weight, even outside of the Genocide route, and the world reacts to your violence accordingly. Every character, no matter their actions in the past, has some type of redeemable quality, and every character deserves to be saved, even if some are harder to reach than others.
In contrast, the sadnesses in ISAT are explicitly said to be completely non-sentient - taking them down is a net positive because it would be far too depressing to slice down dozens of real people for hundreds of Loops of the amount of people they're capable of hurting. It's a similar approach to the King: killing him is a necessary evil. Yes, he had humanizing qualities - redeemable ones, even - but he wasn't willing to be redeemed. The King was the one who took advantage of the party's forgiveness, and no matter his motives, the King was the one who chose to be a monster.
That being said, I do find it funny how different the lines of no return are in ISAT vs UT. Like, Siffrin considers hurting a child the worst thing you could do ever, to the point where they're visibly more distressed over Bonnie's death than anything bad that happens to him personally ever. Meanwhile child endangerment is practically a requirement to be a main character in UT. Most of the time you can't befriend the monsters if they haven't tried to kill you, a CHILD, at least once! Hell, Asgore has killed/caused the death of multiple children, and he's still treated like a surrogate father figure for Frisk by most of the fanbase (including me).
Why am I going on about this? Oh you know...
Reasons ^_-
I just think it's a wonderful idea!